The present invention relates to a network for distributing high-frequency bi-directional signals in a data-processing installation.
The network of the invention for distributing bi-directional signals is formed by a coaxial cable which provides a series supply to a plurality of connection points for the connection of pieces of data-processing apparatus capable of multiple and diverse functions. The link between a piece of apparatus and the coaxial cable is provided at the connection point by a matching device which in the case of reception converts the bi-directional signals into binary signals compatible with the technical characteristics of the logic circuits of the piece of apparatus connected to the connection point and which in the case of transmission converts the binary signals emitted by the piece of apparatus connected to the connection point into bi-directional signals. In the context of the invention a bi-directional signal has the following characteristics:
(1) it is able to assume alternately two voltages of opposite sign on either side of a middle voltage level; PA1 (2) the transmission of binary data by means of bi-directional signals takes place in such a way that a number "1" expressed in binary code is always associated with a voltage which is positive or negative with respect to the middle voltage of the bi-directional signal and that a binary "0" is always associated with the middle voltage of the bi-directional signal; PA1 (3) two successive "1's" expressed in binary code are transmitted in the form of two signals of opposite polarity in bi-directional transmission.
The present-day trend in producing equipment for transmitting pulses by cable between transmitter and receiver apparatus is to electrically isolate the earth or ground circuits of the pieces of apparatus from one another, the chief advantage of which is to improve the signal-to-noise ratio in transmission. Two types of isolation techniques are used at present, the first of which isolates by means of an electro-optical arrangement, and the second of which uses a pulse transformer to separate the transmitter electrically from the receiver.
A first example of matching between a cable and a receiver is disclosed in a French patent application, published as No. 2170990, corresponding to U.S. application Ser. No. 225,793 filed Feb. 14, 1972 now abandoned in which is described transmission-line matching by means of an electro-optical arrangement. The arrangement which is described allows a transmission rate of 20,000 bauds over a distance of the order of a kilometer.
A second example of matching between a cable and a receiver makes use of an inductive wound transformer at transmission and reception, is disclosed in a French patent application published in France on Sept. 21, 1973 as No. 2172154, and is readily adapted to telephone lines. The transformer used is provided with a magnetic core whose hysteresis loop is practically rectangular and thus allows the magnetic core to be switched from a first state to a second state as dictated by the value of the binary signal applied to the primary winding of the transformer.
When the frequency of the transmitted signals is very much higher than that referred to in the latter two French patent applications, the arrangements described in the two applications can no longer be used. In effect, in the case of signals having a frequency of the order of 1 MHz, electro-optical coupling cannot be used because of its narrow pass-band. It is likewise impossible to make use of transformer coupling by exploiting the changeover of its magnetic core between two states, because this causes instability in the transmission line due to the multiple reflection of the signal, which is very difficult to bring under control.
The matching of the present invention employs a transformer for coupling to the transmission line. This coupling differs from the transformer coupling described in the second of the French patent applications mentioned above by virtue of the fact that the hysteresis loop of its magnetic core is not rectangular and that the magnetic core operates on the linear part of its hysteresis loop. The inductance of the transformer is sufficiently great for the magnetising field to be obtained by a minimum of primary current, thus giving low "back-swing" levels. A definition of "back-swing" may be found on page 125 of the book "Electronique des Impulses", Volume 1, by Georges Metzger and J. P. Vabre, pub. Editions Masson et Cie.
Despite the advantages of operation achieved with low "back-swing" levels, nevertheless there are certain drawbacks to using such a transformer to couple a coaxial cable transmission line to a transmitter or a receiver. The resultant increase in the primary inductance of such a transformer increases the sum of the capacitances between the turns of the windings of the primary and secondary. This presents a loading impedance on the line, formed by a capacity representing the sum of the inter-turn capacities and which is connected in parallel with the inductance of the primary and in parallel with the impedance of the active load. Assuming the impedance of the active load to be very high in comparison with the primary inductance and the capacity, the impedance of the active load may be ignored. The same applies for the inductance of the primary if its impedance is very high in comparison with the sum of the inter-turn capacities, and a transmission line which is fed by a source E via a pulse transformer as defined above and which is connected to an active load of very high impedance via a transformer having the same characteristics as that defined above may therefore be considered as a line which is fed directly with the supply voltage E and whose load is a capacity C representing the sum of the inter-turn capacities of the primary and secondary windings of the transformer.
It is known that a transmission line whose load is an impedance other than its characteristic impedance is subject to multiple reflections. In accordance with conventional transmission line theory, the general equation which defines the law governing the change in potential gradient V at a point X along a transmission line is of the form: ##EQU1## which in the case of a loss-free (R = 0 and G = 0) line may be written: ##EQU2## If one defines ##EQU3## where U is the speed of propagation and .delta. the propagation constant, equation (2) becomes ##EQU4## It admits as an integral EQU V(X,t) = g(x - Ut) + h (X = Ut) ##EQU5##
It can be demonstrated that the voltage and current waveform at any point whatsoever along the line is the result of two moving waves which are propagated in opposite directions at the same speed U.
For further details reference may be made to the work by Messrs. Georges Metzger and Jean Paul Vabre, entitled "Electronique des Impulsions", Vol. II, page 21.
The equation for the incident wave is EQU V.sub.i = g(x - Ut) (3) ##EQU6##
The equation for the reflected wave is EQU V.sub.r = h(X + Ut) (5) ##EQU7##
Thus, at each point where a transformer is connected to the transmission line, there will be a mis-match which will produce from an incident wave a transmitted wave which continues to be propagated in the direction of the incident wave, and a reflected wave which is propagated in the opposite direction. If there are a plurality of connection points along the line, there will be a plurality of successive mis-matches, and the transmitted wave from a mis-matched point will behave like an incident wave which, at the next mis-matched point, will in turn produce another transmitted wave and another reflected wave. Thus, if there are n mis-matched points, there will be n waves reflected to each point where a transformer is connected, and the voltage level of the reflected waves may be comparable to the level of the n useful signals. It is necessary to take precautions in the reception circuits to avoid any errors in interpretation.
Hereinafter reflected signals on a capacitive loaded line will be referred to as "capacitive echos". The capacitive echos may be separated from the useful signals by adding at the end of the line a passive filter of the first order. If the capacitive echo is likened to a signal of the form U.sub.O Exp -t/.tau. and if this signal is applied to the input of a low-pass filter whose transfer function is ##EQU8## the response from the output of the filter will be of the form ##EQU9## which is the Laplace function converted from ##EQU10## by taking P = 1/.theta..
If the time constant of the passive low-pass filter is large in comparison with .tau., the amplitude of a filtered capacitive echo decreases markedly and the decay voltage persists for a longer time.
If a stepped signal is applied to the low-pass passive filter, the output response from the filter is of the form E(1 - Exp -t/.theta.).
The addition of the low-pass passive filter is thus a satisfactory solution to the problem of separating the useful signal from the capacitive echo, but it solves only part of the problem and it is in fact necessary to match the line and the low-pass filter to the circuit for shaping and converting the bi-directional signals to give binary signals. It is necessary that the stages for shaping the received pulses should not interfere with the transmission line. Threshold detectors cannot be inserted immediately beyond the passive low-pass filter, since changeover devices of this nature draw a high current at the time of changeover, which would interfere with the low-pass filter and thus with the line. The idea has been considered of connecting the secondary of the transformer to the input of an operational amplifier connected as a voltage follower whose pass-band is twenty times larger than that of the useful frequency of the signal and whose input impedance is thus very high, the low-pass filter being connected directly to the output of the operational amplifier. This solution is also unacceptable since the output pulses are distorted in comparison with the input signal both in respect of amplitude and duration. The input signal saturates the first stage of the operational amplifier and its output voltage cannot catch up with the input signal. Equally unpromising results have been obtained by inserting the passive low-pass filter between the secondary of the transformer and the input of the operational amplifier.
The problem could no doubt be solved with a voltage-following differential operational amplifier having a pass-band very much wider than 20 MH.sub.z, but this would be a very cumbersome solution.